This invention relates to an improved printed radiating element antenna, and most particularly, to a novel slot antenna structure with integral feeding means and array arrangements formed therefrom.
In designing an antenna for radio frequency energy it is important that the antenna be compatible with the feeding network, that is, the transitional device that is to be employed between the antenna element and the feed means to excite the element should be one with little or no discontinuity that would cause bandwidth restrictions.
In seeking a broadband antenna compatible with a feed network, light in weight, rugged in construction and yet simple to construct, the choices available to an antenna engineer are rather limited. Seemingly, a possible candidate having relative good broadband characteristics would be the so-called dual-ridge antenna for transmitting and receiving electrical signals. In general, such an antenna may comprise a ground plane with a pair of matching directional elements or ridges that may extend perpendicularly from a ground plane and have facing inner curved surfaces which converge toward the grounded plane and terminate at a predetermined distance from the ground plane and from each other. At a point of minimum separation between the matching directional elements a transmission line may be readily utilized to excite the matching elements, generally by means of a coaxial feed assembly. It is generally known that when such an assembly or transition is used as a feed line to such a dual-ridge type antenna there may be some discontinuity, in practice, that may often limit or alter electrical characteristics, especially the antenna's bandwidth. Moreover, a dual-ridge antenna is not generally a structure that lends itself to a multiple connection feeding networks as would be necessary in a conformal array structure. Further, dual-ridge antennas with associated transitional devices are generally more difficult to manufacture in a reliable and consistent fashion.
In designing an antenna along with any necessary impedance-matching or power-dividing circuit component associated therewith, an antenna designer must make the antenna perform a desired electrical function which includes, among other things, transmitting/receiving linearly polarized, right-hand circularly polarized, left-hand circularly polarized, etc., r.f. signals with appropriate gain, bandwidth, beamwidth, minor lobe level, radiation efficiency, aperture efficiency, receiving cross section, radiation resistance as well as other electrical characteristics.
It is advantageous for an antenna structure to be lightweight, simple in design, inexpensive and unobtrusive to the environment since the antenna is often required to be mounted upon or secured to a supporting surfaces, such as are often associated with a motorized vehicle, high velocity aircraft, missile, or rocket device which cannot, of course, tolerate excessive deviations from an aerodynamic geometry. Of course, it is also sometimes desirable to conceal or hide an antenna or an array so that its presence is not readily apparent for security as well as aesthetic purposes. Accordingly, the ideal antenna should physically be very thin and not protrude on an external side of a mounting surface, such as an aircraft skin or the like, while yet still exhibiting all the requisite electrical characteristics.
Antennas having very low profiles which can be flush mounted on a supporting surface are generally referred to as conformal antennas. As mentioned, such an antenna conforms to the contour of its supporting surface, and, therefore, reduces or eliminates any turbulent effects that would result when such a device is mounted or secured, for example, to a vehicle and propelled through space. Conformal antennas may, of course, be constructed by several methods, but can be generally produced by rather simple photoetching techniques well-known in the art. Such techniques offer ease of fabrication at a relatively low production cost. Briefly, conformal antennas or printed circuit board antennas, as they may be called, are formed by etching a single side of a unitary metallically clad dielectric sheet or electrodeposited film using conventional photoresist-etching techniques. Typically, the entire antenna structure may possibly be on 1/32 inch to 1/8 inch thick which minimizes cost and maximizes manufacturing/operating reliability and reproducibility.
It can be appreciated that the cost of fabrication of such printed circuit board antennas is substantially minimized since single antenna elements and/or arrays of such elements together with appropriate r.f. feedlines, phase shifting circuits and/or impedance matching networks may all be manufactured as one integrally formed electrical circuit by using low cost photoresist-etching processes commonly used to make electronic printed circuit boards. This method of producing an antenna structure is to be compared with the often complicated and costly prior art techniques for fabrication of antennas for achieving polarized radiation patters as, for instance, a turnstile dipole array, the cavity backed turnstile slot array and other special antennas.
Antennas of the type considered herein, viz., flared notch type antenna, have been configured in various forms. Briefly, U.S. Pat. No. 2,942,263 to Baldwin teaches a conventional notch antenna device. Further, U.S. Pat. No. 2,944,258 to Yearout, et al., discloses a dual-ridge antenna as previously disclosed having a broad bandwidth. U.S. Pat. No. 3,836,976 to Monser, et al., discloses a broadband phased array antenna formed by pairs of mutually orthogonal printed radiating elements, each one of such elements having a flared notch formed thereon. Monser et al., teaches a feed means in the form of a coaxial cable that is soldered to a metallization layer, this may generally cause some discontinuity which often limits the bandwidth of an antenna. On the other hand, U.S. Pat. No. 4,500,887 to Nester discloses a broadband radiating element designed to provide a smooth, continuous transition from a microstrip feed configuration to a flared notch antenna.